The file would be encrypted using a symmetric encryption scheme (like AES), and then the key for decrypting it would be encrypted to each recipient. Adding an extra recipient then just involves encrypting the 128 or 256 bit key, which should be negligible increase in size.
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Generally, one would use a Hybrid Encryption scheme: http://en.wikipedia.org/wiki/Hybrid_cryptosystem
The file would be encrypted using a symmetric encryption scheme (like AES), and then the key for decrypting it would be encrypted to each recipient. Adding an extra recipient then just involves encrypting the 128 or 256 bit key, which should be negligible increase in size.